Most plants synthesize a type of protein whose main function is to provide a store of nitrogen, sulphur and carbon. The majority of these storage proteins are synthesized in organs of reproduction, propagation and dispersal, such as seeds, pollen, spores and tubers. Storage proteins can be classified according to their extraction and solubility in water (albumins), dilute aqueous solutions of salts (globulins), alcohol/water mixtures (prolamines) and dilute acids or alkalis (glutelins) (P. R. Shewry, Biol. Rev. 70:375-426 (1995)). Seeds of cereal plants contain large amounts of storage proteins, most of which fall into the prolamine and globulin classes. Prolamines are the major seed storage proteins in all cereals except oats and rice. The major seed storage protein in oats and rice are globulins. Prolamines are given different names in the different plants: secalins in rye, hordeins in barley, zeins in maize and gliadins and glutenins in wheat. Prolamines are characterized by their high contents of proline and amide nitrogen and low levels of charged acidic and basic amino acids. Beyond these broad similarities, the cereal prolamines vary widely in their amino acid compositions and sequences. Oat and rice seeds differ from wheat, barley and rye in that they contain relatively small amounts of prolamines, called avenins in oats. Oat seeds store 10-20% by weight prolamines and 75-80% by weight globulins.
The glutenins, which include both high molecular weight (HMW) glutenin subunits and low molecular weight (LMW) glutenin subunits, comprise an economically important class of wheat seed storage proteins. The apparent molecular weights of the individual HMW glutenin polypeptides or subunits range from 90 to 200 kDa. These subunits crosslink by disulfide bonds among themselves and with LMW glutenin polypeptides to form polymers exceeding one million daltons in molecular weight. HMW glutenins constitute 8-10%, while LMW glutenins constitute 15-20% of the total endosperm protein. Both HMW and LMW glutenin proteins play important functional roles in determining the end-uses of wheat flour.
In wheat, HMW glutenins are encoded at the Glu-1 loci on the long arms of the group 1 chromosomes. Each locus consists of two separate genes, encoding an x-type and a y-type subunit, respectively. These pairs have never been confirmed to be separated by recombination. This has made determination of their separate contributions to bread dough properties difficult to assess by genetic correlation studies. For a review of the genetics and biochemistry of glutenin polypeptides, see, Shewry et al., J. Cereal Sci. 15:105-120 (1992).
Both the quantity and identity of specific HMW glutenin alleles contribute to the differences in bread-making quality of various cultivars. For instance, deletion of glutenin genes results in a decrease in the overall levels of HMW glutenins, which results in decreases in bread-making quality (see, e.g., Lawrence et al. J. Cereal Sci 7:109-112 (1988)).
The effects of overproducing HMW glutenin on protein accumulation and baking quality has not been assessed because such lines of wheat have not been found among natural populations. In addition, direct alteration of the glutenin subunits that form the polymers is not possible using standard breeding methods. Thus, the art lacks reproducible and efficient methods of producing lines with altered glutenin contents.
Wheat is one of the most important crops in the world, due to its ability to be grown in a wide range of local conditions and due to the unique properties of wheat flours and doughs. Wheat flours and doughs allow processing into a wide range of food products including various types of breads, pasta and noodles, cakes and biscuits. Glutenin proteins are largely responsible for the visco-elastic properties that confer the unique processing properties to wheat doughs. Shewry et al. (J. Sci. food Agric., 73:397-406 (1997)) reports transforming tritordeum with the high molecular weight (HMW) subunit glutenin genes. Tritordeum is a novel cereal produced by crossing pasta wheat and a wild barley, and lacks the D genome associated with bread making quality in wheats. As yet, there are no published reports of non-wheat plants being transformed with glutenin genes. Therefore, the effects of glutenin on the processing characteristics of flour from non-wheat plants has not been assessed. The present invention addresses these and other needs.